1. Field of the Invention
The present invention relates to a flat package integrated circuit device.
2. Description of the Prior Art
Integrated circuit devices, in their initial state, are in the form of tiny chips a few millimeters square having two faces, one of which is defined as the working face. The working face incorporates the electrical circuits and carries output pads which are suitable for connection to conductors. Connection is usually made by soldering, after which the devices may be encapsulated.
Encapsulation protects the devices mechanically and gives them a conventional format which makes them easier to use in complicated electronic structures. Fabrication of such devices necessarily involves the application of each device to a supporting member, the use of output terminals external to the package, the connection of these external terminals to respective output pads of each device via at least one array of conductors, and the encapsulation of the assembly as a whole so as, in particular, to improve its mechanical, thermal and electrical characteristics.
The flat package integrated circuits which have been most widely used in the electronic arts for more than a decade are in the form of a right-angled parallelepiped, bordered longitudinally by two opposing rows of connecting leads which form the output terminals of the package. Because of the arrangement of these leads, the package is normally referred to as DIL ("Dual-In-Line") for DIP ("Dual-In-Line Package"). In actual fact, these leads are the outer ends of converging conductors each terminating at an output pad of the device. The set of conductors, together with the integrated circuit device, are embedded in a plastic or ceramic encapsulation which forms the package itself.
DIL packages currently on the market have a thickness of several millimeters (generally 3 to 4 mm) and an area of at least a square centimeter. However, given the incessant demand for greater miniaturization in many fields of application, manufacturers are being forced to reduce the size of packages. Although it may be possible to cut down the area of DIL packages without any disadvantage this is not true with respect to thickness. To this end, the encapsulation must in fact form a rigid cladding for the two large faces of the integrated circuit chip so that, under flexion, it will not fracture, or, in particular, damage its circuits, and plastic materials and resins are sufficiently rigid only when of relatively large thickness. Thin layers of ceramic material on the other hand crack under flexion. In theory, ceramics should in fact be usable at thicknesses above 0.635 mm but in practice it has been found that the minimum thickness is slightly less than a millimeter. Thus, plastic and resins are not suitable for encapsulation of extremely thin packages and ceramics are generally limited for use at thicknesses of one millimeter and above.
To overcome these disadvantages, manufacturers have replaced the encapsulation, in its capacity of a rigid protective wall, by a metal reinforcement. The non-working face of the device rests in the bottom of a thin metal tray through which pass connecting leads which are insulated from the tray by glass. The inner ends of these leads are connected to the output pads on the working face of the device by flexible wires (using the technique known as wire-bonding) or by beam leads (using the technique known as T.A.B. or "tape automatic bonding"). An encapsulating substance is often used to form the assembly into a solid unit and/or a cover is used to close the tray. It will be apparent that the production of such a package, which is known by the name "flat pack", is very complicated and therefore extremely expensive as compared with a DIL package because of the fact that it is necesary to have a metal tray, to form lateral openings in the tray to form beads of glass in the respective openings and insert the connecting leads in them before the glass beads solidify, to lay down the integrated circuit device in the bottom of the tray, to solder the wires or beam leads to the connecting leads and to the output pads of the device by respective ones of their two ends, to cast the encapsulating substance in the tray and/or to cover the whole article with a cover or a molding.
This form of package has been improved to give the "package carrier". In this design, the tray and the cover, if there is one, are made of ceramic material, which is a good dissipator of heat and relatively malleable. The output terminals which pass through the tray are cast into the ceramic and appear on the outside as lateral pads rather than leads. The package can be connected to an external mounting by insertion in a seating complementary to the surface of the package which is provided with corresponding pads to make contact with those of the package. In one particular known instance, the external mounting is a DIL package in which the "package carrier" type package takes the place of an integrated circuit device.
Although less expensive and smaller in area than the previous package, this package is necessarily thicker since the ceramic material must form a relatively thick layer if it is to have the required rigidity.
Furthermore, large scale integration, LSI, which is currently becoming increasingly widespread in the manufacture of integrated circuits, poses another problem. The concentration in one and the same substrate of circuit components performing different functions means that the resulting device is confined to very specific applications and this, generally speaking, prevents the device in question from being mass-produced. It would be advantageous if a plurality of interconnected, mass-produced integrated circuit devices could be combined, as required by the customer, into a single package which was as compact as possible. The advantages which would be gained are the use of unspecialized low cost devices, the wide choice offered to customers by the combining of the devices, and the possibility of combining integrated circuit devices employing different technologies.
For example, regarding this latter point, at the present time any monolithic integrated circuit device incorporating a memory and a control circuit for the memory is very restricted in its capabilities and it is therefore necessary to use two devices, which perform the memory and memory control functions, respectively, and to interconnect them in order to obtain an assembly of acceptable performance.
The structures of the packages which have been described above are not such as to allow a plurality of devices and the circuit for interconnecting them to be incorporated in one and the same package due to the fact that this would call for the output conductors to cross the interconnecting conductors.
This difficulty has been overcome by replacing, in a package of the flat pack or package carrier type, the integrated circuit device by a carrier wafer containing the integrated circuit devices, their interconnecting circuit, and the output conductors, in the form of printed circuits. The output conductors terminate in connecting islands to which linking conductors leading to the output terminals of the package are soldered.
It was seen above that the thickness of such packages cannot in practice be reduced below a millimeter and that they are more or less complicated and costly to produce. In addition, since the carrier wafer has to rest in the protective tray forming part of the package, the interconnecting circuits and integrated circuit devices must all be carried on the same face of the wafer. Consequently, the interconnecting circuits can only be fitted together or overlapped by spreading them out over a plurality of electrically isolated conductive layers. Ceramic material was the natural choice for the material to form the wafer in view of its good physical electrical and thermal properties, its low cost, the expertise with which printed circuits and soldered joints can now be formed on it, and the advantage offered by the nature of the package that only one face of the wafer is used. However, it is known that the ceramic material must be thick if all the advantages listed above are to be obtained.
In certain particular applications, it is found that the thickness of a package containing one or more integrated circuit devices is a factor of prime importance. This is the case, for example, with the manufacture of standardized portable cards of the credit card type, such as are described in French patent specification No. 2,337,381, corresponding to U.S. Ser. Nos. 751,954 and 4,588 assigned to the assignee of the present invention. The dimensions of such cards are governed by standard ISO/DIS 2894 issued by the International Standards Organization. The cards must therefore be in the form of a rectangle measuring 85.59.+-.0.12 mm.times.53.97.+-.0.06 mm and must have a thickness of 0.76.+-.0.08 mm, which can be increased by a maximum of 0.05 mm to allow, for example, the name and address of the holder of the card to be shown by means of applied items (such as sticky labels) or by embossing the card itself. To put into practice the invention described in the above patent specification it is therefore necessary for a package which is to be accommodated in a cavity in a card to have a thickness of less than a millimeter. In addition, since the cards are made of a plastic material, usually polyvinyl chloride (PVC), it is also necessary for the package to be relatively flexible, without this flexibility prejudicing the structure or operation of the electrical circuitry which it incorporates. Assuming that the package is to contain a memory component to record the identity of the holder of the card, his confidential code, and the debiting and crediting operations which he is allowed to perform, and a component to be used to control the said memory, the multi-component package described above is quite incapable of meeting the conditions laid down for the credit card. It has also been shown that even if a single monolithic device performing both a memory function and a memory-control function were incorporated in a known single-component package, the package would still be too thick and unusable for standard credit cards.
In other applications, the factor of prime importance will be the area or volume taken up by the package, which will be the crucial factor in deciding whether to use it. However, if the package is to contain a plurality of integrated circuit devices, the surface area required to fit them to one face of the wafer contained in the tray of the package is necessarily large. In other words, the restrictions imposed by the structure of the package are irreconcilable with the demands of size reduction.